1. Field of the Invention
This invention relates to turbine machines and particularly to coolable rotor blades in turbine machines.
2. Description of the Prior Art
A gas turbine engine is typical of turbine machines in which the concepts described herein may be advantageously employed. In a gas turbine engine pressurized air and fuel are burned in a combustion chamber to add thermal energy to the medium gases flowing therethrough. The effluent from the chamber comprises high temperature gases which are flowed downstream in an annular flow path through the turbine section of the engine. Nozzle guide vanes at the inlet to the turbine direct the medium gases onto a multiplicity of rotor blades which extend radially outward from the engine rotor. The nozzle guide vanes and rotor blades are particularly susceptible to thermal damage and are commonly cooled to control the temperature of the material comprising the components in anticipation of high effluent temperatures. Cooling air from the engine compressor is bled through suitable conduit means to the turbine for subsequent distribution to the rotor blades and guide vanes.
Thermal degradation of the component material due to excessive temperatures is one principal problem which heretofore has adversely limited the service life of turbine rotor blades. In the shank portion degradation due to creep phenomenon is of primary concern as elongation of the blade shank initially alters the radial position of the blade and may ultimately result in fracture. Control of the shank material temperature is required to limit elongation and to prevent creep from becoming a seriously limiting factor in rotor blade service life. The airfoil portion of each blade is similarly susceptible to creep although the centrifugally generated forces decrease in the radial direction outwardly to the blade tip. Additionally, the airfoil portion of each blade is highly susceptible to erosion as the local surface temperature of the blade approach the melting point of the component material.
The service life of rotor blades is also adversely limited by thermal fatigue. Thermal fatigue results from the mechanical effect of repeated thermal stresses occurring during nonuniform expansion of the various regions of the blade. Repeated thermal cycling precipitates substantial stress excursions and results in ultimate failure of the component. Control of the thermal gradients which produce stress excursions is required to mitigate the harmful effects of changing thermal environments.
Continuing efforts are underway to improve the service life of turbine rotor blades by limiting the temperature of the component material in the various regions of each blade and by reducing the thermal gradients across each blade.